THE  UNIVERSITY 

OF   ILLINOIS 

LIBRARY 


A6RICDLTURAL 
LIBRARY 


UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.  213 


TWO  ILLINOIS  RHUBARB  DISEASES 

BY  FRANK  LINCOLN  STEVENS 


URBANA,  ILLINOIS,  JANUARY,  1919 


CONTENTS  OF  BULLETIN  No.  213 

PAGE 
RHUBARB  AKTHRACNOSE 299 

General  Character    299 

The  Fungus  300 

Isolation 301 

Culture  Characters 301 

Inoculations 305 

Germination  of  Conidia 305 

Taxonomy 305 

RHUBARB  LEAF  SPOT 308 

The  Fungus  310 

Isolation 312 

Culture  Characters   312 

Taxonomy 312 


TWO  ILLINOIS  RHUBARB  DISEASES 

By  FRANK  LINCOLN  STEVENS,  PROFESSOR  OF  PLANT  PATHOLOGY,  UNIVERSITY 

OF   ILLINOIS1 

RHUBARB   ANTHRACNOSE 

(Due  to  Collet otriclium  erumpens) 

Attention  was  first  called  to  this  disease  on  market  rhubarb  in  the 
stores  of  Champaign  during  the  spring  of  1918.  It  has  since  been 
repeatedly  seen  in  the  grocery  stores  and  has  been  collected  in  the  field 
in  numerous  localities,  in  both  cases  in  such  abundance  as  to  show 
that  it  constitutes  a  real  factor  in  rhubarb  culture. 

GENERAL  CHARACTER 

The  disease,  which  is  of  fungous  origin,  consists  of  a  soft  rot  of  the 
petioles.  As  yet  no  natural  field  infection  of  other  parts  of  the  plant 
has  been  seen.  The  decayed  spots  usually  are  soft,  watery,  and 
translucent,  oval  in  outline,  with  the  long  axis  lengthwise  of  the 
petiole  (Fig.  1).  When  they  attain  a  length  of  somewhat  more  than 
a  centimeter,  the  black,  sporulating  bodies  (acervuli),  smaller  than  fly 
specks,  appear  abundantly  in  the  centers  of  the  spots  (Fig.  2).  In 
advanced  stages  the  whole  petiole  is  covered  with  acervuli  and  is 
entirely  soft  and  rotten  (Fig.  3). 

In  the  market  usually  only  the  milder  cases  of  disease  are  found ; 
petioles  with  numerous  small  rotten  spots  which  have  escaped  the 
notice  of  the  grower  when  preparing  his  product  for  sale.  In  the 
field,  often  all  old  petioles  lying  dead  on  the  ground  are  thickly  cov- 
ered with  acervuli,  and  older  petioles  slightly  wilted  are  heavily 
infected.  In  general  this  disease  appears  much  more  abundantly  on 
old  plant  parts  than  on  fresh,  vigorous  parts,  tho  many  vigorous, 
salable  petioles  are  attacked,  as  is  clearly  evident  from  the  abundance 
of  the  disease  in  the  market  as  well  as  in  the  field. 

This  disease  has  been  collected  at  Champaign  and  Urbana,  Cham- 
paign county ;  at  Kankakee  and  Bourboiinaise,  Kankakee  county ;  and 
near  Anna,  Union  county;  and  is  apparently  of  quite  general  oc- 
currence. 

The  injury  done  is  threefold.  Even  a  small  amount  of  the  dis- 
ease in  a  bunch  of  market  rhubarb  practically  destroys  the  possibility 
of  selling  the  bunch  to  a  careful  purchaser.  In  the  field  many  petioles 


Assisted  in  the  laboratory  and  in  illustrating  by  Nora  E.  Dalbey. 

299 


300 


BULLETIN  No.  213 


[January, 


otherwise  salable  must  be  discarded  on  account  of  the  rot.  The  pre- 
mature death  of  infected  petioles  and  consequently  of  the  leaf  blades 
lessens  the  general  strength  of  the  plant. 

THE  FUNGUS 

The  rotten  spots  are  uniformly  occupied  by  a  species  of  Colletot- 
richum,  and  in  early  stages  of  decay  by  this  fungus  alone.  Young, 
watery,  translucent  spots  which  have  not  yet  developed  acervuli  show 
the  diseased  tissue  quite  fully  occupied  by  the  mycelium.  This 
mycelium  is  hyaline  and  of  somewhat  characteristic  appearance  in  that 


FIG.  1. — PETIOLES  OF  EHUBARB  SHOWING  VARIOUS  STAGES  OF  DECAY  EESULTING 

FROM  COLLETOTRICHUM 

the  older  parts  are  rather  coarse  (4  p.),  conspicuously  septate,  and 
constricted  at  the  septa  (Fig.  4).  Younger  threads  of  mycelium  are 
smaller,  tapering  to  2  /u  at  the  extremities. 

The  acervuli.  begin  subcuticularly  as  an  aggregate  of  hyphae, 
Fig.  5  (1),  which  soon  ruptures  the  cuticle.  When  mature  they  are 
circular  or  oval  and  of  an  average  diameter  of  about  200  p..  Soon 
after  rupturing  the  cuticle,  one  or  more  setae  appear,  and  in  the 
mature  acervulus  the  setae  are  numerous,  usually  from  fifteen  to 


1919] 


Two  ILLINOIS  EHUBARB  DISEASES 


301 


Fig.  2. — A  single  spot, 
enlarged.  The  minute 
pustules  are  the  acer- 
vuli 


twenty  or  more  Fig.  5  (2,  3).  Spores  begin 
to  form  as  soon  as  the  cuticle  ruptures,  and 
the  mature,  undisturbed  acervulus  is  covered 
with  a  mound  of  spores,  often  so  deep  as  to 
cover  even  the  tips  of  the  setae. 

The  setae  are  black,  somewhat  pale  at  the 
tip,  usually  3-  or  4-septate,  upward  of  150  p. 
long,  and  about  7  p.  thick  at  base,  and  with  an 
acute  tip,  Fig  6  (1,  2). 

The  conidia  are  falcate  (Fig.  7),  quite  uni- 
form in  size,  measuring  22  to  29  /*  by  3.5  p., 
hyaline  and  continuous,  and  acute  at  each 
end.  The  conidiophores  are  shorter  than  the 
conidia,  simple  and  continuous. 

Isolation 

The  fungus  was  readily  isolated  by  sev- 
eral of  the  usual  methods: 

1.  By   dilution   platings   of  spores.     In 
many   cases  the  plates  were  entirely  pure, 
with  hundreds  of  colonies   of  the   Colletot- 
richum. 

2.  By     differential    growth,     by    direct 


planting  of  bits  of  infected  tissue  upon 
poured  corn-meal  agar  plates.  When 
this  method  was  employed,  the  Colletot- 
richum  outgrew  its  contaminators,  and 
could  thus  be  easily  isolated. 

3.  By  direct  planting  of  spores  from 
drops  of  sterile  water  upon  poured  agar 
plates.  The  colonies  so  secured  on  corn- 
meal  agar  did  not  produce  normal  acer- 
vuli,  but  they  did  eventually  produce 
characteristic  conidia,  and  wrhen  trans- 
ferred to  sterile  rhubarb  gave  abundant 
normal  acervuli. 

Culture  Characters 

On  corn-meal  agar  plates  the  myce- 
lium showed  a  tendency  to  darken  in 
some  cases,  producing  many  filaments 
almost  black,  and  giving  to  all  the  col- 
onies a  dark  appearance.  Analysis 
shows  that  the  darkening  of  the  colonies 
is  due  to  an  excessive  production  of  ap- 


Fig.  3. — Advanced  stages  of  de- 
cay of  the  rhubarb  petioles 


302 


BULLETIN  No.  213 


[January, 


pressoria1  and  of  structures  which  are  morphologically  appressoria. 
Often  these  are  produced  in  large  numbers  in  clumps  which  appeal- 
as  black  sclerotia-like  bodies,  several  millimeters  in  diameter.  This 
was  particularly  true  where  two  colonies  approached  each  other  in 
the  plate  (Fig.  8).  The  structure  of  the  component  parts  of  such 
a  spot  is  represented  in  Fig.  6  (3).  A  general  view  of  several  col- 
onies on  corn-meal  agar  is  shown  in  Fig.  8.  On  this  medium  the 
acervuli  are  produced  somewhat  sparsely,  after  an  interval  of  about 
a  week. 


FIG.   4. — CHARACTERISTIC   BRANCHING   AND   SEPTATION   OF  THE   COLLETOTRICHUM 

MYCELIUM 


When  sterile  (autoclaved)  rhubarb  was  laid  upon  poured  plates 
of  corn-meal  agar  and  then  inoculated  with  the  fungus,  acervuli 
formed  much  more  quickly  than  upon  corn-meal  agar  alone,  and  spores 
were  produced  in  much  greater  quantity.  Thus  abundant  mature 
acervuli  were  present  upon  the  bits  of  rhubarb  at  the  end  of  three 
days. 

Bits  of  rhubarb  petioles  were  cut  aseptically,  and  placed  on  poured 
corn-meal  agar  plates.  Upon  these  growth  was  as  upon  the  auto- 
claved rhubarb  but  decidedly  more  vigorous. 

Corn-meal  agar  was  poured  into  plates,  then  inoculated.  When 
the  colonies  attained  a  diameter  of  about  a  centimeter,  bits  of  raw, 
aseptic  rhubarb  were  laid  on  the  agar  about  a  centimeter  from  the 
edge  of  the  colony,  in  order  to  see  whether  the  growth  rate  in  this 
sector  of  the  colony  would  be  altered  by  the  rhubarb.  No  such  effect 
was  noticeable,  but  it  was  found  that  tho  the  fungus  made  acervuli 


'Hasselbring,    H.   Bot.    Gaz.   42,    135,    1906;    and   Halsted,   B.D.,    N.J.    Agr. 
Exp.  Sta.  Ept.,  1892,  303. 


1919] 


Two  ILLINOIS  RHUBARB  DISEASES 


303 


but  very  sparsely  on  the  agar  it  did  make  them  abundantly  upon 
the  bits  of  raw  rhubarb  upon  the  agar. 

On  various  autoclaved  plugs  the  fungus  grew  well :  e.g.,  on  Bumex 
crispus,  with  typical  acervuli  but  growth  rather  slow;  on  Rumex 
acetosella,  similarly  but  growth  more  vigorous ;  on  Polygonum  erectum 
growth  was  scant  and  aeervuli  small;  on  Polygonum  lapatliifolium 
growth  was  similar  to  that  on  Polygonum  erectum  but  slightly  more 
vigorous;  on  Tropoeolum  stems,  growth  was  very  rapid  and  acervuli 


FIG.  5. — (1)  A  VERY  YOUNG  ACERVULUS;  AN  AGGREGATE  OP  MYCELIUM  BELOW 
THE  CUTICLE  WITH  INDICATIONS  OF  BEGINNINGS  OF  SETAE.  (2)  TYPICAL 
GROUP  OF  ACERVULI.  (3)  A  TYPICAL  ACERVULUS 

numerous,  showing  these  steins  to  be  more  favorable  than  any  other 
medium. 

On  cabbage  plugs  the  whole  culture  was  less  dark  than  on  rhu- 
barb plugs,  the  floccose  aerial  mycelium  was  white,  and  there  was 
almost  entire  absence  of  appressoria,  and  of  sclerotia,  or  other  dark 
structures  except  those  of  the  acervuli,  which  were  very  abundant 
and  normal. 


304 


BULLETIN  No.  213 


[January, 


On  rice  there  was  an  unusually  large  production  of  appressoria 
and  sclerotia,  giving  the  culture  a  striking  black  character  (Fig.  9). 
In  tubes  on  autoclaved  rhubarb  plugs,  growth  was  vigorous,  the  sur- 
face of  the  plugs  becoming  closely  covered  with  acervuli  which  pro- 
duced spores  about  a  week  after  inoculation.  In  the  older  portions 


FIG.  6. — (1)  DETAIL  OF  SETAE  AND  CONIDIOPHORES.     (2)  A  SINGLE  SETUM.     (3) 
COMPONENT  PARTS  OF  THE  SCLEROTIA-LIKE  BODIES  PRODUCED  IN  AGAR  PLATES 

the  plugs  were  covered  by  a  thin  growth  of  aerial  mycelium.  On 
corn-meal  agar  in  slant  tubes  the  surface  of  the  slant  soon  became 
covered  with  black  sclerotia-like  structures,  and  a  ring  of  these  struc- 
tures was  formed  wherever  the  fungus  touched  the  glass. 


1919] 


Two  ILLINOIS  EHUBARB  DISEASES 


305 


Inoculations 

When  spores  from  diseased  tissue,  or  fragments  of  diseased  tissue, 
or  mycelium   from   a   pure   culture   were  inoculated   into   rhubarb 

petioles,  the  typical  rot  rapidly  followed. 
Tho  the  disease  was  not  found  in  the 
field  upon  leaf  blades,  laying  the  spores 
upon  leaf  blades  in  a  moist  chamber  in  the 
laboratory  resulted,  in  about  seven  days, 
in  large,  rotten,  leaf  spots  bearing  numer- 
ous typical  acervuli.  The  disease  also  often 
appeared  thru  natural  infection  on  leaf 
blades  in  the  laboratory. 


Fig.  7. — Spores  of  Colletot- 
richum 


Germination  of  Conidia 

Germination  in  water  in  hanging  drop 
gave  the  usual  characters  of  the  anthrac- 
nose  fungi;  i.e.,  the  spores  usually  became 

two-celled,  developed  a  germ  tube  from  one  or  both  cells,  and  often 
from  the  germ  tubes  one  or  more  appressoria  (Fig.  10). 

TAXONOMY 

The  fungus  is  a  typical  Colletotrichum.  Never  were  setae  absent 
from  mature  acervuli,  nor  is  there  any  basal  tubercular  structure 
which  would  throw  it  into  the  genus  Volutella.  Colletotrichum  as 


FIG.  8. — TYPICAL  COLONIES  ox  CORX-MEAL  AGAR 
LEFT:  FOUR  COLONIES  EIGHT:  MANY  COLOXIES 

A   dark  line   due   to   sclerotia-like   bodies   is   evident   in   all   cases  when   one 
colony  meets  another. 


306 


BULLETIN  No.  213 


[January, 


given  by  Saccardo1  contains  113  form  species. 
Of  these,  however,  only  33  have  falcate  spores. 
There  are  83  species  of  Volutella  listed  by 
Saccardo.  The  falcate-spored  Colletotrichums 
are  given  in  the  accompanying  table,  together 
with  such  of  the  closely  related  genus  Volu- 
tella as  have  falcate  spores. 

From  this  tabulation  have  been  omitted 
such  forms  as  are  described  as  having  spores 
"straight  or  slightly  curved,"  "straight  or 
curved,"  or  similarly  designated,  on  the  as- 
sumption that  such  are  not  typically  falcate, 
a  character  which  were  it  present  would  be 
sufficiently  obvious  to  be  mentioned. 

Accepting  the  descriptions  as  correct,  Nos. 
1  to  27  inclusive  and  Nos.  29,  38,  and  43 
do  not  agree  in  spore  size  with  the  fungus 
under  consideration.  Nos.  44  to  47  inclusive 
need  not  be  considered.  Nos.  28,  30  to  37, 
and  39  to  42  are  so  close  in  spore  measure- 
ments to  the  species  causing  the  rhubarb  rot, 

that  this  might  indeed,  on  that  basis  alone,  be  considered  as  co-specific 
with  any  one  of  them ;  nor  is  there  any  evidence  from  measurements 
of  setae,  except  in  the  case  of  No.  37,  that  would  bar  such  an  assump- 
tion. Close  inspection  of  the  specific  descriptions  eliminates  Nos. 
40  and  41,  which  are  clearly  tubercular. 

There  then  remains  a  group  consisting  of  ten  species  which  from 
the  descriptions  are  indistinguishable  from  each  other  and  from  the 


Fig.  9.— Growth  of  Col- 
letotrichum  on  auto- 
claved  rice 


FIG.  10. — GERMINATING  CONIDIA  AND  APPRESSORIA 


1Saceardo.  Sylloge  Fungorum. 


1910] 


Two  ILLINOIS  EHUBARB  DISEASES 


307 


FALCATE-SFORED  FORMS  OF  COLLETOTRICHUM  AND  VOLUTELLA 
Arranged  in  order  of  maximum  spore  length  (/*) 


No. 

Saccardian 

Name 

Spore 

Setae 

Host 

Vol.    No. 

Length  X  Breadth 

Length  X  Thickness 

1 

2 
3 

4     3234 
11     3676 
10     6851 

V.  chalybea 
C.  sanguisorbae 
C.  sphaeriaeforme 

7-8X4-5 
9-10X2-2.5 
10  X31 

115-140X2.3 
80-10.0X3-6 

"fimo" 
Sanguisorbia 
Menispermum 

4 
5 

6 

7 
8 

4     3237 
14     4026 

10     6846 
14     4019 
4     3227 

V.  buxi 
C.  yuccae 

C.  pisi 
C.  solitarium 
V.  comata 

10-11X3^ 
13X3.5 

11-13X3-4 
12-14X2.5-3 
12-14  X 

100-120X4 
60X6 

60-  90X6 
65-  75X2-3 

Buxus 
Yucca 

Pisum 
Solidago 
Robinia 

9 
10 

18     5034 
16     3885 

C.  liliacearum 
C.  piri 

12-17X2.5-3.5 
13-18X3.5-5.5 

70X5-5.5 

Hemerocallus 
Pirus 

11 
12 
13 

3     3843 
4     3263 
18     5014 

C.  volutella 
V.  therryana 
C.  theobromicolum 

14-18X2.5-3.5 
15-18X3 
15-18X4-5 

300X5-6 
150X1 

Ulmaria 
Grasses 
Theobroma 

14 
15 

16 
17 

18 

16     3882 
18     5772 

10     6852 
16     3886 
10     6854 

C.  violae-tricoloris 
V.  tristis 

C.  spinaciae 
C.  elasticae 
C.  gloeosporioides 

20X5 
11-20X2 

14-20X2.5-3 
16-20X4.5-5 
18-20X5-6 

20-  70  X 

400X7-8 

60-  75X4-4.5 
200-250  X 

Viola 
Erica 

Spinacia 
Ficus 
Hedera 

19 

16     4258 

V.  violae 

15-21X3-4 

Viola 

20 

21 
22 
«8 

4     3244 

16     4262 
10     6847 
22     7733 

V.  fusarioides 

V.  allii 
C.  carpophilum 
C.  (impelinum 

18-21X2-3.5 

18-21X3-4 
16-22X2.5-4 
17-22X4-5 

60-  72X5-7 

60-175X6 
60-100X5-6 

Citrus 

Allium 

Astragalus 
Vitus 

24 
25 

26 
27 

*8 

16     3883 
22     7767 

11     3679 
22     7741 

10-   6845 

C.  rhoinum 
C.  vermicularia 

C.  ailanthi 
C.  fructus 
(V.  fructus} 
C.  brassicae 

20-22X4-4.5 
20-22X4 

22X4-5 
17-23X2.5-3.5 

19-24  X 

150-180 
80-  90X4 

90-135X5-9 
100-4"OOX5-8 

Rhus 
Bromus 

Ailanthus 
Pirus 

Brassica 

29 

30 

31 
32 
33 
34 
35 

22     7763 
22     7744 

3     3842 
11     3683 
22     7732 
22     7768 
11     3681 

C.  sublineola 
C.  hedericola 

C.  erumpens 
C.  falcatum 
C.  vitis 
C.  cereale 
C.  omnivoriim 

15-25X4-5 
17-25X3.5 

25X5 
25X4 
21-25X2.5 
18-26X3-4 
20-28x3-5 

'30-  50X7 
140X7 

150X4 
100-200X4 
140-160X6-9 
60-120X6-8 

Sorghum 
Hedera 

Ruscus 
Saccharum 
Vitis 
Grasses 
Funkia,  Aspedis- 

36 
37 

38 
39 
40 

41 
42 
43 
44 

22     7753 
3     3844 

22     7765 
18     5025 
14     4493 

14     4494 
18     5036 
22     7766 
4     3245 

C.  platani 
C.  lineola 

C.  antarcticum 
C.  fici-elasticae 
V.  acalyphae 

V  .  oxyspora 
C.  andro'pogonis 
C.  janczewslcii 
V  .  isabcllina 

25-28X3-5 
25-28X3.5-4 

15-30X3-4 
20-30X3-4 
25-30X2-3 

25-30X2-3 
30X4 
24-34X3-6 

200 
60X3 

30-100X4-6 
200X6 
150-210X4-7 

100-150X4-5 
100-150X8 
70-150X4 

tra,  etc. 
Platinus 
Andropogon, 
Umbellifers 
Poa 
Ficus 
Acalypha 

Beta 

Andropogon 
Poa 
Salix 

45 

4     3232 

V  '  .  pulchella 

Branches 

40 

4     3252 

V  '  .  melaloma 

Carica 

47 

4     3226 

V  .  georginae 

Georgina 

'Spuriously  2  to  3  septate. 


308 


BULLETIN  No.  213 


species  described  in  this  paper.     These  are  designated  as  follows, 
f  rom  their  oldest  species,  as  the — 

Colletotrichum  erumpens  group: 

No.  31.     Colletotrichum  erumpens  Sacc. 
28.     C.  brassicae  Schulz  and  Sace. 

35.  C.  omnivorum  Hals. 

32.  (7.  falcatum  Went. 
39.     G.  fici-elasticae  Zimm. 

33.  C.  vitis  1st. 

42.  C.  andropogonis  Zimm. 

30.  C.  hedcricola  Laub. 

34.  C.  cereale  Manns 

36.  C.  platani  Da  Camara 

In  the  present  condition  of  knowledge  concerning  these  forms, 
and  until  monographic  work  is  completed  regarding  them,  it  seems 
wise  to  assume  that  the  rhubarb  Colletotrichum  belongs  to  this  group, 
and  tentatively  to  refer  to  it  as  Colletotriclmm  erumpens. 

RHUBARB  LEAF  SPOT 

(Due  to  Phyllosticta  straminella) 

This  disease  was  first  collected  at  Bourbonnaise,  Kankakee  county, 
Illinois,  where  it  was  in  great  abundance  in  one  field.  Nearly  every 
leaf  in  the  plantation  was  affected  with  many  spots,  often  occupying 
considerably  more  than  half  of  the  leaf  area.  A  general  idea  of  the 


FIG.  11.— A  SINGLE  DISEASED  SPOT  OF  THE  PHYLLOSTICTA  DISEASE  ox  RHUBARB 


1919] 


Two  ILLINOIS  RHUBARB  DISEASES 


309 


nature  of  the  effect 
upon  the  leaf  is 
given  in  the  front- 
cover  illustration. 
More  detail  of  a 
single  spot  is 
shown  in  Fig.  11. 
Unlike  the  preced- 
ing disease  this  is 
not  preeminently  a 
disease  of  old 
leaves  but  even  the 
c  o  mp  aratively 
young  leaves  may 
be  seriously  spot- 
ted. While  mainly 
affecting  the  leaf 
blade,  it  has  also 
been  found  upon 
the  petioles.  Upon 
the  leaf  blade  the 

chief  character  is  the  irregularly  circular  dead  spot,  varying  from 
a  few  millimeters  up  to  several  centimeters  in  diameter.  The  dead 
area  is  tan  color  and  often  dry  and  cracked  or  torn  (see  front  cover). 
The  margin  of  the  spot  is  definite,  with  a  rather  sharp  limitation 
between  diseased  and  healthy  tissue.  Close  inspection  shows  numer- 


FIG.   12. — AN   ENLARGED  VIEW   OF  A   PORTION   OF   THE 

SPOT  SHOWN  IN  FIG.  11 
The  pycnidia  are  here  clearly  shown. 


FIG.  13. — PHYLLOSTICTA.    A  PYCNIDIUM  IN  SECTION,  SHOWING  SUBCUTICULAR  DE- 
VELOPMENT AND  SHAPE  OF  THE  PYCNIDIUM 
The  cavity  was  entirely  filled  with  spores. 


310 


BULLETIN  No.  213 


Fig.  14. — Phyllosticta,  Pycnidia 
viewed  from   above 


ous  very  minute 
dark  pustules 
(pycnidia) 
(Figs.  11  and 
12).  On  petioles 
and  stems  the 
spots  are  oval 
1  en  gthwise  of 
the  petiole,  dis- 
tinctly sunken, 
and  quite  dry. 
S  om  e  times  a 
spot  extends  for 
nearly  the  whole 
length  of  a  peti- 
ole, occupying 
p  e  r  haps  only 
one  side  or  part 
of  one  side  of  it. 
In  addition  to 
collections  at 


Fisr. 


15. — Phyllosticta     spores 
from  a  leaf  spot 


Fig. 


16. — Phyllosticta 
small  type 


spores, 


Bourbonnaise,  others  have  also  been  made  at  Urbana,  Kankakee,  and 
Champaign.  Specimens  collected  at  Ladoga,  Indiana,  in  1916,  by  Dr. 
P.  J.  Anderson  and  at  Racine,  Wisconsin,  by  M.  W.  Gardener,  bear  the 
same  fungus.  When  prevalent  to  the  extent  shown  in  the  cover  illus- 
tration, and  to  that  degree  on  many  leaves,  as  is  the  case  in  some  fields, 
the  drain  on  the  vigor  of  the  plants  must  be  large. 

THE  FUNGUS 

Microscopic  examination  invariably  revealed  the  presence  of  a 
pycnidial  fungus  of  the  Phoma  or  Phyllosticta  type.    Fig.  12,  an  en- 


FIG.  17. — PORTION  OF  PETRI  DISH  SHOWING  PHYLLOSTICTA  COLONIES 
This  plating  was  made  direct  from  rhubarb  leaves. 


1919] 


Two  ILLINOIS  RHUBARB  DISEASES 


311 


FIG.  18. — A  SINGLE  COLONY  OF  THE  PHYLLOSTICTA  ON  CORN-MEAL  AGAR 


FIG.  19. — PORTION  OP  PETRI  DISH  SHOWING  SEVERAL  COLONIES  OF  THE 

PHYLLOSTICTA 

larged  view  of  a  single  spot,  shows  the  numerous  pycnidia.  These 
are  approximately  circular,  tan  colored,  about  120  to  150  /x  in  diameter, 
with  an  ostiole  20  to  30  p.  wide  (Fig.  14).  Under  appropriate  con- 
ditions the  spores  issue  in  cirrhi  or  long  agglutinated  coils.  From 
the  leaf  spot  the  spores  are  rather  large  for  a  Phyllosticta  ( 10  to  19  p 
by  4  to  5/x),  and  oblong  (Fig.  15)  and  straight  or  very  slightly  curved. 
Specimens  from  the  petiole  and  from  some  cultures  to  be  described 
later  gave  a  much  smaller  type  of  spore,  as  small  as  4  to  5  //,  by  1.5  to 
5  /x  (Fig.  16). 


312  BULLETIN  No.  213  [January, 

Isolation 

The  fungus  was  isolated  on  corn-meal  agar  in  all  of  the  ways 
noted  on  page  301,  and  with  the  same  general  results ;  i.e.,  the  fungus 
grew  readily  and  normally  in  culture  (Fig.  17).  Photographs  of 
plates  and  colonies  are  given  in  Figs.  17,  18,  and  19. 


On  plates  on  corn-meal  agar,  the  fungus  grows  rapidly  and  pro- 
duces pycnidia  quickly.  When  thickly  sown,  sporiferous  pycnidia  are 
produced  in  thirty-six  hours ;  when  more  thinly  sown,  pycnidia  are  not 
so  quickly  produced.  The  colonies  eventually  become  quite  dark  in 
the  central  region.  The  pycnidia  arise  uniformly  by  the  simple  meri- 
stogenous  method.  There  is  in  corn-meal  agar  a  very  remarkable  change 
in  spore  size  from  that  exhibited  by  the  same  fungus  when  in  its 
natural  habitat.  On  this  medium  the  spores  are  very  rarely  as 
long  as  7  p.  (Fig.  16).  In  very  old  cultures  spores  of  somewhat  larger 
size  are  found,  but  none  as  large  as  on  the  original  leaf  spots.  On 
various  autoclaved  vegetable  plugs  the  fungus  grew  well  and  formed 
numerous  pycnidia  normally.  As  with  the  Colletotrichum,  Tropceolum 
stems  were  the  most  favorable  medium.  On  cabbage  the  growth  was 
exceptionally  white.  On  rice  the  whole  culture  became  black  and 
was  nearly  like  that  of  Colletotrichum  (cf.  Fig.  9). 

The  conidia  germinated  readily  in  hanging  drops  of  water,  nearly 
every  spore  germinating.  There  were  formed  no  appressoria  such  as 
were  invariably  formed  under  similar  conditions  by  the  Colletotri- 
chum. 

TAXONOMY 

Regarding  this  as  a  Phyllosticta  and  its  spore  measurements  to 
be  the  extremes  noted  above;  i.e.,  4  to  19  /x  by  1.5  to  5  /x,  there  are 
829  species1  listed  by  Saccardo  from  which,  on  a  basis  of  spore  size, 
it  would  be  indistinguishable.  Accepting  the  narrower  limits  of  spore 
measurement  obtained  from  the  large  spored  forms  from  foliar  mate- 
rial ;  i.e.,  10  to  19  /x  by  4  to  5  /*,  there  are  48  species  listed  by  Saccardo 
from  which  it  could  not  be  distinguished  by  spore  measurements. 
One  only  of  these,  however,  occurs  on  any  of  the  Polygonaceae.  Ten 
other  species  of  Phyllosticta  are  listed  by  Saccardo  on  Polygonaceae, 
but  none  of  these  has  spores  as  large  as  the  large-spored  form  described 
in  this  paper.  All  of  these,  with  the  exception  of  one,  have  maximum 
lengths  of  12  /x,  while  in  most  of  them  the  maximum  length  is  below 
8  11.  The  one  large-spored  species,  P.  straminella  Bres.,  described  on 
Rumex  acestosa,  is  given  as  having  spores  12  to  20  p.  long  and  is  suf- 
ficiently close  in  morphology  to  the  form  under  consideration  to  be 
regarded  as  the  same  species. 


lrriiese  facts  are  taken  from  a  tabulation  of  the  species  of  Phyllosticta  listed 
by  Saceardo,  prepared  for  future  publication  by  Mrs.  E.  Young  True. 


UNIVERSITY  OF  ILLINOIS-URBAN* 


